1. Field of the Invention
The present invention relates to a multi-mode, 2-stroke/4-stroke internal combustion engine operation which maximizes efficiency and power, while minimizing emission.
2. Description of Related Art
Spark Ignition (SI) engines, which are widely used in United States in passenger vehicles, suffer from low efficiency in comparison to Compression Ignition (CI), or Diesel engines, especially in a partial load due to throttling. CI engines, however, exhibit high particulate and NOx emissions characteristics due to their combustion nature. Recently, Homogeneous Charge Compression Ignition (HCCI) engines have been introduced, which have higher efficiency comparable to CI engines as well as minimum particulate and NOx emissions characteristics, with the versatility of using gasoline as well as diesel fuel.
In HCCI engines, a spark plug or a high-pressure injector is not used for initiation of the ignition of the fuel; instead, auto-ignition of the fuel (either gasoline or diesel) and air mixture at the end of the compression stroke is achieved by providing an elevated starting temperature at the beginning of the stroke. This elevated temperature is achieved mostly by two ways: heating the intake air or using the exhaust gas from the previous cycle. In the latter system, one can re-induct or trap the hot exhaust gas from the previous cycle by changing the valve timings.
The amount of the exhaust gas trapped in HCCI engines is usually about 50% in mass of the total gas inside the cylinder. Although this exhaust gas increases the mixture temperature before combustion, it actually decreases the peak temperature after combustion due to dilution effect. As a result, the NOx emission, which is exponentially proportional to the gas temperature, is about two orders of magnitude lower than that in conventional SI or CI engines. One can also achieve higher efficiency comparable to CI engines due to the dethrottling of the intake manifold and combustion shape closer to ideal Otto cycle.
However, the limited work output is one of the major challenges in HCCI operation, which limitation is caused by the high dilution of mixture in the cylinder. The dilution with hot exhaust gas is not only required to increase the mixture temperature to achieve auto-ignition, but also required to limit the high rate of pressure rise, which otherwise would be destructive to the engine components. In this case, almost half of the cylinder is filled with the exhaust gas, which results in producing approximately half of the work output compared to that in SI of the same engine volume.
To overcome the load limitation, current state of art utilizes a hybrid of HCCI/SI or boosted HCCI. In a hybrid approach, a mode switching from SI to HCCI occurs when the low load is required where SI has poor efficiency (see, e.g., U.S. Pat. Nos. 6,390,054 B1, and 6,742,494 B2). However, in this case, the emission and efficiency benefits of HCCI operation are lost in the medium to high load range, and the transient combustion control of HCCI in mode switching is not a subtle issue in current research and industries. Secondly, boosted HCCI may give higher power, but it is also limited since the combustion becomes too noisy and destructive due to the high rate of pressure rise from rapid burn rate.